Electrophoretic display device

ABSTRACT

An electrophoretic display device for generating a colored image has pixels each having a medium ( 12 ) that is substantially transparent in the optical spectral range and at least two types of independently controllable electrophoretic particles ( 31 C,  31 M,  31 Y). The particle species absorb certain wavelengths whereas they are transparent (and basically non-scattering) for light that is absorbed in other particle species. By moving these particles in/out a visible area of the pixel by generating electric fields color absorption can be controlled at will. As the particles are basically transparent (at least for the colors absorbed in the other particle species), the full pixel area (and volume) can be used for any particle species. This way the maximum brightness can be achieved for any color.

The invention relates to an electrophoretic display device forgenerating a colored image, the device comprising picture elements whichhave a visible area, a medium that is substantially transparent in theoptical spectral range, at least two types of independently controllableelectrophoretic particles, and means for controlling the distribution ofthe particles in the medium.

An electrophoretic display device is known from patent applicationWO99/53373. The display device comprises picture elements, each elementhaving several types of particles in a suspending fluid, e.g. as shownin FIGS. 5 to 8 of the document. The particles are charged and aretherefore controllable by an electric field, which effect is called theelectrophoretic effect. The suspending fluid functions as a medium thatis permits the particles of different types to travel at differentspeeds under the influence of the electric field. The movement of eachtype of particles is controlled by providing electrodes and a suitablepattern of control voltages. Selected particles are moved to a visiblearea of the picture element. Each type of particles has a specificcolor, and therefore a specific type of particles at the front surfaceof the picture element in the visible area result in a specific color ofthe picture element. In the example shown in FIG. 6 the picture elementcomprises red (R), green (G) and blue (B) particles. The particles aresupposed to be negatively charged, and are first moved to the back endof the picture element by an electric field in a direction transverse tothe visible area by a positive voltage between a bottom electrode and atop electrode. Then the voltage is reversed, and the particles start tomove to the front of the picture element. The red particles are thefastest, so they arrive first. The voltage is then removed from theelectrodes, and the red particles are visible because the red componentfrom the incident white light is reflected. The remaining part of thelight is absorbed so that the blue and green components of the incidentlight cannot reach the blue and green particles which are floating lowerin the picture element. Different colors can be generated by moving allparticles to the front and thereafter removing faster particles to thebackside of the picture element. For example yellow could be generatedby moving a mix of red and green particles to the front. A problem ofthe known display is that contrast and brightness of the colorsgenerated are not satisfactory.

Therefore it is an object of the invention to provide a display devicehaving a higher brightness and contrast.

The object is achieved with a device as defined in the opening paragraphwhich comprises at least one reservoir located outside the visible areafor containing particles that are not in the visible area, each particletype being substantially transparent for a first part of the opticalspectral range, being substantially non-back scattering, and beingabsorbing or reflective for a second part of the optical spectral range,the first and second part together covering the optical spectral range,and the second parts of the optical spectral range of said particletypes being substantially non-overlapping. The effect is that particlesof a specific type are obstructing substantially no light outside thespecific second part of the spectral range. The light produced by thepicture element in the specific second part of the optical spectralrange is controlled by moving particles of that type to the visiblearea. The intensity of all second parts of the optical spectral rangecan be controlled independently, while no light is absorbed in parts ofthe spectral range by particles that are needed for controlling otherspectral ranges. The advantageous effect is that the brightness andcontrast for all colors will be high.

The invention is also based on the following recognition. The inventorhas seen that the known multicolored picture elements can produce mixedcolors only at a reduced brightness. For a primary color, i.e. a colorthat is produced by a single particle type, a high brightness may beachieved because all incident light of that primary color will bereflected by the particles at the front surface of the picture element.However, a mixed color such as yellow in a system having red, green andblue particles is produced by having 50% green and 50% red particles atthe surface of the visible area. Hence only 50% of the incident light isreflected and the brightness for yellow is reduced to 50%. The inventorhas concluded that the known system is flawed because it uses particlesthat obstruct, i.e. absorb or scatter light outside the part of thespectrum that is controlled. Particles that do not obstruct prevent thereduced brightness, provided that they are removed from the visible areato the reservoir when they are not needed.

In an embodiment of the device the non-overlapping second parts togethersubstantially cover the optical spectral range. The advantage is thateach picture element can produce all colors of the optical spectralrange at maximum brightness, because all light within the opticalspectral range can be controlled.

In an embodiment of the device the particles are substantiallynon-scattering due to at least one of the following: due to having arefractive index substantially equal to the refractive index of theelectrophoretic medium, due to the size of the particles being smallerthan the wavelength of light in the optical spectral range, due to thesize of the light absorbing layers/structures in the particle beingbelow an internal wavelength in the particle, the internal wavelengthbeing the wavelength of light in the optical spectral range divided bythe refractive index of the particle. The advantage is that the contrastis enhanced by the particles being non-scattering.

In an embodiment of the device the picture elements comprise colorfilters in which each picture element color filter absorbs or reflectsat least one filtered part of the optical spectral range while theremaining non-filtered part of the optical spectral range substantiallycorresponds to the range of the optical spectral range covered by saidsecond parts of the particle types in the respective picture element.The advantage is that several colors can be produced using a combinationof color filters and fewer types of particles within the medium.

Further preferred embodiments of the device according to the inventionare given in the further claims.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 a shows a prior art picture element of a reflective display inreset mode,

FIG. 1 b shows a prior art picture element of a reflective display inred display mode,

FIG. 2 a shows a display having obstructing particles in red displaymode,

FIG. 2 b shows a display having obstructing particles in yellow displaymode,

FIG. 3 a shows a display having non-obstructing particles in red displaymode,

FIG. 3 b shows a display having non-obstructing particles in yellowdisplay mode,

FIG. 4 shows three cooperating picture elements having color filters,and

FIG. 5 shows a picture element having a color filter.

The Figures are diagrammatic and not drawn to scale. In the Figures,elements which correspond to elements already described have the samereference numerals.

FIG. 1 shows a prior art electrophoretic display picture element indifferent states. FIG. 1 a shows a prior art picture element of areflective display in reset state. The picture element 13 (also calledpixel) is seen from a viewing direction 10. A medium 12 compriseselectrophoretic particles indicated by the characters R, G, B indicatingthe colors red, green and blue. Each particle type reflects a particularcolor, and absorbs the remaining part of the optical spectrum. A topelectrode 11 and a bottom electrode 14 are provided for controlling themovement of the colored particles in the depth direction indicated by‘d’ transverse to the visible area. The electrodes 11,14 do not blockthe visible area of the picture element. The particles are supposed tobe negatively charged, and are moved to the bottom end of the pictureelement 13 by an electric field in a direction transverse to the visiblearea by a positive voltage between the bottom electrode 14 and the topelectrode 11. All particles R, G, B are moved to the bottom of thepicture element 13 for achieving the reset state. The medium usually istransparent, and therefore in the reset state light of all colors willbe reflected resulting in a white color being produced by the pictureelement. The medium in this type of pixel may also be colored. FIG. 1 bshows a prior art picture element of a reflective display in red displaystate. Selected particles R are located at the visible front surfacearea of the picture element 13. The specific type R of particles at thefront surface of the picture element in the visible area results in thespecific color red of the picture element. The red state is achieved asfollows. After the reset state the voltage is reversed, and theparticles start to move to the front of the picture element. Particleshave different speeds of movement at certain strength of the electricfield. The red particles R are the fastest, so they arrive first at thefront surface. The voltage is then removed from the electrodes, and thered particles are visible because the red component from incident whitelight is reflected. The remaining part of the light is absorbed so thatthe blue and green components of the incident light cannot reach theblue and green particles which are floating lower in the pictureelement. Different colors can be generated by moving all particles tothe front and thereafter removing faster particles to the backside ofthe picture element.

FIG. 2 a shows a display having obstructing particles in red displaymode. FIG. 2 b shows the same picture element in yellow display mode.The Figure is intended to explain the negative effect of usingobstructing particles. White light 21W is shown to be incident on thevisible area 23 of the picture element. A reservoir 24 is shown on theleft side of the picture element, which reservoir is not visible for theviewer, e.g. by providing a black mask above the reservoir 24. A topelectrode 25 and a bottom electrode 26 are positioned in the reservoirto control the movement of the particles. A top electrode 27 and abottom electrode 28 are positioned in the visible area 23 to control themovement of the particles. Positive and negative voltages are applied tothe various electrodes 25,26,27,28 to independently control the movementof the different types of particles. For good contrasts the pixelpreferably is placed on a black background. In FIG. 2 a the pictureelement is shown in red display state. Red light 21R is shown to bereflected by the red particles 22R, which are located in the visiblearea. The blue particles 22B and the green particles 22G are located inthe reservoir. In FIG. 2 a the picture element is shown in yellowdisplay state. Red light 21R and green light 21G is shown to bereflected by the red particles 22R and 22G, which are located in thevisible area. The blue particles 22B and some green particles 22G andred particles 22R are located in the reservoir. Half of the viewing areais covered with red and the other half with green particles. In thiscase the brightness for yellow is 50% of the maximum achievable yellowbrightness if all red and green light would have been reflected. It isto be noted that a random distribution of particles 22R and 22G at theinterface would produce the same effect.

Particles used in existing electrophoretic display concepts obscureother particles, and colors are made by having different particlespecies near the top electrode where there is a limited viewing areaavailable. In electrophoretic displays based on obscuring particles itis not possible to produce any color over the full pixel area. Thisresults in a considerable loss of (color) brightness.

FIG. 3 a shows a display having non-obstructing particles in red displaystate. FIG. 3 b shows a display having non-obstructing particles inyellow display state. FIG. 3 shows a picture element according to theinvention. The pixel of the electrophoretic display has threenon-obstructing particle species in a suspending medium. The medium mayfor example be a transparent fluid or a gas. The particle species areabsorbing for red indicated as cyan 31C, absorbing green indicated asspecies magenta 31M and absorbing blue indicated as yellow 31Y, whereaseach species is basically transparent for the light absorbed in otherspecies. The pixel has a reservoir 24 that is not visible to theobserver. The pixel has a visible area which is visible to the viewerand has a reflecting background 34. FIG. 3 a shows the red state wherethe full visible area can contain both green 31M and blue 31Y absorbingparticles. FIG. 3 b shows the yellow state where the pixel volumecontains blue absorbing particles 31Y. In both cases 100% of the maximumachievable brightness is obtained for the colors produced. Note that inFIG. 3 a the same optical perception of the pixel would result when theparticle species are distributed more randomly throughout the pixelvolume. It is to be noted that all picture elements shown in FIGS. 3 and4 have a configuration similar to FIG. 2, including the visible area 23,the reservoir 24 and electrodes 25,26,27,28. The picture elementsaccording to the invention may have a reflector and be used in areflective system as shown in FIGS. 3 and 4, but alternatively be usedin a transmissive system without a background reflector, e.g. using abacklight unit.

The electrophoretic display according to the invention uses particlespecies that absorb (or reflect) certain wavelengths whereas they aretransparent (and basically non-scattering) for light that is absorbed inother particle species. It is noted that back-scattering is to beprevented, but some degree of forward scattering can be beneficial forthe contrast. Further it is to be noted particles in this document alsoincludes (micro-) emulsions, i.e. the suspending medium comprises smalldroplets of a second liquid medium, which droplets areelectrophoretically controllable. By moving the particles in/out a pixelvolume, color absorption can be controlled or switched on and off atwill. As the particles are basically transparent (at least for thecolors absorbed in the other particle species), the full pixel area (andvolume) can be used for any particle species. This way the maximumbrightness can be achieved for any color. The particle species are movedelectrophoretically into the pixel volume from a connecting reservoir(area or volume) or vice-versa. In the reservoir particles are notvisible to the observer. The reservoir is adjacent or below the pixelvolume. In an embodiment the reservoir is common for all particlespecies; alternatively each particle species can have its own specificreservoir area or volume.

The suspending medium is transparent so that in the absence of particlesthe observer sees the background through the pixel volume. Thebackground can be a white diffuser, a rough reflector or, when combinedwith a front scattering film, a specular reflector.

The table below gives an example of desired optical properties for anelectrophoretic display with three particles: particle species absorbingfor transparent for 1 red green, blue 2 green red, blue 3 blue red,greenThis set of three particle species results in a full color pixel byusing a reservoir from which the particles are drawn into the pixelvolume. Without particles, the background is visible through the pixelvolume. The background is reflecting for all colors. The reflection canbe specular, diffuse or any combination.

-   -   In the dark state, all incoming light must be adsorbed and all        particle species have to be present in the pixel volume. In case        the absorption bands of the particle species do not fully        overlap, top filters or an additional ‘black’ absorbing particle        species can be used to enhance the contrast. The ‘black’        particles are absorbing in a substantial part of the optical        spectral range, including an overlap with at least some of the        second parts.    -   The white state is produced by removing all particles from the        pixel volume into the reservoir so that the background becomes        visible through the pixel volume.    -   Any color can be made in the electrophoretic pixel by mixing the        right amount of the various particle species in the pixel        volume.

FIG. 4 shows three cooperating picture elements having color filters.The left pixel has a cyan filter 41C, the middle pixel has a magentafilter 41M, and the right pixel has a yellow filer 41Y. The threesub-pixels have CMY color filters combined with three different kinds oftwo-particle electrophoretic sub-pixels based on non-obstructingparticle species and constitute a full color system. Particles in thereservoir to the left of the visible area having the filter are notvisible to the viewer. Below the pixel volume a reflecting background isplaced. The blue state can be achieved in only two of the three subpixels. In the yellow color filter element 41Y on the right, blue lightis absorbed and the corresponding pixel is switched to the dark(absorbing) state in order to obtain a saturated blue for the viewer.Hence, ⅓ of the display area cannot be used for blue. The same appliesfor other colors. Therefore, the total color brightness is 67% of themaximum achievable color brightness. However each sub-pixel only needsto control two types of particles, which is easier to manufacture andfaster in response.

In an embodiment electrophoretic pixels are combined with a spatiallyseparated subtractive color filter. In one specific embodiment only twoparticles species are required per pixel to produce a full color displayas shown in FIG. 4. The optical properties of the particles are chosendepending on the color absorbed in the overlying color filter element.The table below gives an example for particles that can be combined inan electrophoretic pixel underlying a cyan color filter element. displayelement absorbing for transparent for cyan color filter element redgreen, blue particle 1 green (red) blue particle 2 blue (red) greenThe cyan color filter element always absorbs red light so thecombination given in the table cannot yield a red pixel. Magenta andyellow color filter elements can be combined with other electrophoreticpixels to yield red. In the CMY color filter display, only two out ofthe three CMY color filter elements can be used to yield a certainprimary color (red, green, blue). This means that ⅓ of the incominglight is lost for color production. Hence, a color brightness of ⅔ ofthe maximal color brightness can be achieved, and it requires only twoparticle species.

When using non-obstructing particles, the whole pixel volume determinesthe optical perception of a pixel. The perception is rather independentof the internal distribution of the particle species in the pixelvolume. Random and ordered distributions may give the same opticalappearance.

The reservoir can be partially shielded from the pixel volume by meansof a wall. Each pixel can have its own reservoir but one reservoir canalso serve several pixels. Electrode structures in the reservoir andpixel allow selectively transporting particle species from the reservoirinto the pixel volume. The electrode structures can move particlesthrough lateral or transversal fields or any combination. Particles inthe reservoir are not visible to the observer. This can be achieved bymeans of a black mask, by a black reservoir background or by situatingthe reservoir beneath the reflecting background of the pixel. Thereservoir has to be designed in such a way that it occupies a minimalviewing area. Pixel walls can surround pixels as long as these walls doallow for connections to the reservoir(s).

Non-obstructing particle species can be derived along methods known fromthe liquid toner, ink and pigment industry, or from electrophotographictechnologies. Non-obstructing particles that reflect specific parts ofthe optical spectrum are cholesteric flakes. In order for particles tobe transparent for non-absorbed wavelengths, the size of the lightabsorbing layers/structures in the particle should be well below theinternal wavelength if the article (i.e., the wavelength of the lightdivided by the refractive index of the particle). For this purpose small(nanometer sized) particles can be used. Alternatively a larger (porous)particle with a refractive index matched to the solvent can be used.Light absorption can then be achieved by coating the outside or/and inthe pores with a dye layer that is thinner then the internal wavelengthof the light. Suitable pigment particles can be acquired from pigmentindustry, e.g. a diarylide yellow pigment based on dichlorobenzidine(known as Novoperm Yellow HR02 from Clariant), a quinacridone magentapigment (known as Toner Magenta E 02, or PV Fast Red E5B, or PV FastPink E from Clariant), and a phtalocyanine cyan pigment (known as TonerCyan BG from Clariant), all having particle sizes of 50 to 150 nm(typical values).

FIG. 5 shows a picture element having a color filter. Between twotransparent substrates or support plates 50 a suspending fluid andelectrophoretic particles are located. The Figure shows a pictureelement having a magenta color filter element 41M. A reservoir 56comprises a substantially transparent or translucent fluid while theauxiliary reservoirs 57 comprise particles, 58Y and 58C, which absorb atleast blue and at least red respectively. By providing electrodes 51,52, 53 with appropriate voltages the particles 58 move to the reservoir56. The electro-optical components particles, 58Y and 58C) are nowintermixed within the reservoir 56. The auxiliary reservoirs 57 areprovided with a black mask 59. It is to be noted that an embodiment ofthe picture element has no filter element 41M, but only at least twotypes of particles. A number of additional reservoirs may be used fordifferent types of particles. Further details of configurations forcontrolling different types of electrophoretic particles usingreservoirs and electrodes are described in co-pending patent applicationEP 01203176.1 (application number of PHNL 010567).

In further embodiments of the device the particles are not fully mobileor may be partially fixed, for instance a working device can be based ona Cholesteric Liquid Crystal material of two different colors in whichswitching between a transparent or translucent state and a reflectivestate occurs at different voltages for different colors. The CLCmaterial does not necessarily have to be a layer. It can also have theform of encapsulated flakes or particles.

Layers comprising such capsules or other layers comprising(controllable) reflecting or absorbing particles do not need to becompletely transparent, as long as the main direction of the lightpropagation is maintained (translucent or forward scattering layers).Scattering may even be advantageous, provided no significantbackscattering occurs, to obtain better absorption, leading to thinnerlayers. To this end the pixel walls 54 may be provided with a reflectingor absorbing layer.

In the example the color filter part absorb (or reflect) the colorscyan, magenta and yellow. In principle the device can also be based on acolor filter having colors which absorb (or reflect) other parts of theoptical spectral range and adapting the switchable components (layers)accordingly. For example one could use a first sub-pixel having a colorfilter part absorbing in the range 550-650 nm, a second sub-pixel havinga color filter part absorbing in the range 450-550 nm and a thirdsub-pixel having a color filter part absorbing in the range 400-450 nmand 650-7000 nm. The optical spectral range on the other hand mayinclude infrared and ultraviolet. In general the above embodiment of thedevice can be understood as having a color filter in which n sub-pixelcolor filter parts absorber reflect n parts of the optical spectralrange (preferably non-overlapping or having minimal overlap), the devicecomprising (n−1) switchable electro-optical components for controllingabsorption or reflection of the remainder of the spectral range in eachsub-pixel.

Although the invention has been mainly explained by embodiments usingoptical elements using the electrophoretic effect, the invention is alsosuitable for other controllable optical elements such as electrochromicdisplays. It is noted, that in this document the use of the verb‘comprise’ and its conjugations does not exclude the presence of otherelements or steps than those listed and the word ‘a’ or ‘an’ precedingan element does not exclude the presence of a plurality of suchelements, that any reference signs do not limit the scope of the claims,and that several ‘means’ or ‘units’ may be represented by the same item.Further, the scope of the invention is not limited to the embodiments,and the invention lies in each and every novel feature or combination offeatures described above.

1. Electrophoretic display device for generating a colored image, thedevice comprising picture elements which have a visible area, a mediumthat is substantially transparent in the optical spectral range, atleast two types of independently controllable electrophoretic particles,means for controlling the distribution of the particles in the medium,and at least one reservoir located outside the visible area forcontaining particles that are not in the visible area, each particletype being substantially transparent for a first part of the opticalspectral range, being substantially non-scattering, and being absorbingor reflective for a second part of the optical spectral range, the firstand second part together covering the optical spectral range, and thesecond parts of the optical spectral range of said particle types beingsubstantially non-overlapping.
 2. Device as claimed in claim 1, whereinthe non-overlapping second parts together substantially cover theoptical spectral range.
 3. Device as claimed in claim 1, wherein theparticles are substantially non-scattering due to at least one of thefollowing: due to having a refractive index substantially equal to therefractive index of the electrophoretic medium, due to the size of theparticles being smaller than the wavelength of light in the opticalspectral range, due to the size of the light absorbing layers/structuresin the particle being below an internal wavelength in the particle, theinternal wavelength being the wavelength of light in the opticalspectral range divided by the refractive index of the particle. 4.Device as claimed in claim 1, wherein the particles are porous andsubstantially non-scattering due to having a refractive indexsubstantially equal to the refractive index of the electrophoreticmedium and being larger than the wavelength of light in the opticalspectral range, the particle and/or the pores being coated by a dyelayer that is thinner than internal wavelength in the particle, theinternal wavelength being the wavelength of light in the opticalspectral range divided by the refractive index of the particle. 5.Device as claimed in claim 1, wherein the device comprises a backgroundreflector behind the picture elements, the background reflector inparticular being a white diffuser or a rough reflector, or a frontscattering film and a background reflector behind the picture elements,the background reflector being a specular reflector.
 6. Device asclaimed in claim 5, wherein the reservoir is located behind thebackground reflector.
 7. Device as claimed in claim 1, wherein thepicture elements further comprise an additional type of independentlycontrollable electrophoretic particles, which additional type isabsorbing in a substantial part of the optical spectral range, includingan overlap with at least some of the second parts.
 8. Device as claimedin claim L, wherein the picture elements comprise at least twoelectrodes for selectively transporting the particles from the reservoirto the visible area and vice versa.
 9. Device as claimed in claim 1,wherein the picture elements comprise color filters in which eachpicture element color filter absorbs or reflects at least one filteredpart of the optical spectral range while the remaining non-filtered partof the optical spectral range substantially corresponds to the range ofthe optical spectral range covered by said second parts of the opticalspectral range of the particle types in the respective picture element.10. Device as claimed in claim 9, wherein the picture elements comprisen different color filters, n being an integer larger than 1, thefiltered parts of the optical spectral range being substantiallynon-overlapping.